Process and apparatus for the fractional distillation of crude oil

ABSTRACT

A process for the fractional distillation of crude oil, which comprises the steps of: feeding a continuous current of crude oil at 310-400° C. into a turbomixer comprising a tubular cylindrical body ( 1 ) provided with a rotor ( 9 ) formed with helically oriented paddles ( 10 ) and rotatably supported inside the body, along with a continuous current of steam; subjecting both currents to the mechanical action of the paddled rotor ( 9 ), so as to create a thin tubular dynamic turbulent layer, which is discharged and fed continuously into a fractioning column, at a predetermined height level in the column, to produce an upward-flowing vapor current and a downward-flowing liquid current therein; and optionally discharging the downward-flowing liquid current continuously from the column bottom, and feeding it continuously into the turbomixer in the same direction as the crude oil current.

DESCRIPTION FIELD OF APPLICATION

[0001] The field of this invention is the petrochemical industry.

[0002] In particular, the invention relates to a process and anapparatus for fractionating crude oil.

PRIOR ART

[0003] It is well known that to obtain suitable products for use asfuels, raw materials in the chemical industry, lubricants, etc., fromcrude oil, a number of processing steps are to be carried out, startingof necessity with the fractional distillation of the crude oil. This maybe followed by other processes, among which reforming and cracking, forfurther purifying the products delivered from the fractionating process,improving their quality, or modifying them.

[0004] The fractional distillation of crude oil is carried out, in largefractionating columns provided with perforated or bell-shaped trays, byfeeding the crude oil, as preliminarily desalted and heated in a furnaceto a temperature of 310° to 400° C., onto a tray located at anintermediate height level in the column. At that temperature, the crudeoil is a mixture of vapor and liquid. The vaporized low-boilingcomponents flow up through the overlying trays in the column, while thehigh-boiling components flow down through the underlying trays to thecolumn bottom.

[0005] Since skilled persons in the art are surely familiar with theoperation of a fractionating column as a whole, heed will be taken ofwhat goes on at the column bottom.

[0006] Steam is conventionally injected at the column bottom in order tostrip the current of liquid that flows down to the column bottom of anyvolatile components trapped therein, and improve the output inlow-boiling components of the fractionating process.

[0007] As the fractionating process proceeds, the liquid collected atthe column bottom becomes progressively more dense and viscous, turningeventually into a tar-like residue. For this reason, the process must bestopped periodically to allow the column bottom to be cleared of thedeposits and cleaned.

[0008] Of course, stopping the refining process affects the plantthroughput adversely.

[0009] Also, a significant amount of valuable low-boiling componentsremain trapped in the viscous residue, it being impossible to strip themfrom the residue by ordinary steam injection.

[0010] The underlying problem of this invention is to provide a processand an apparatus that can overcome the aforementioned drawbacks of prioroil refining processes.

SUMMARY OF THE INVENTION

[0011] The idea that led to the solution of the above problem was thatof placing the crude oil to be fractionated in the most intimate contactpossible with the stripping steam, in order to increase the yield inlightweight components and to reduce to a minimum the periodic stops forremoving the residue from the column bottom.

[0012] Thus, the technical problem has been solved by a processcomprising the steps of:

[0013] providing a turbomixer, which comprises a cylindrical tubularbody provided with a heating jacket, at least one inlet opening, atleast one outlet opening, and a rotor formed with helically orientedpaddles and rotatably supported inside said body;

[0014] feeding a continuous current of crude oil, as preliminarilydesalted and heated to 310-400° C., into said turbomixer, with theturbomixer wall being heated to the same temperature as the inflowcurrent of crude oil;

[0015] supplying a steam current continuously in the same direction assaid crude oil current;

[0016] subjecting both said currents to the mechanical action of saidpaddle rotor, as driven for a peripheral velocity of at least 15 m/s, soas to intimately blend both currents together, centrifuging the blendedcurrents against said heated wall to create a dynamic thin tubular layerin turbulent motion thereon, and conveying said blended currents to saidat least one outlet opening; and

[0017] continuously discharging said blended currents and feeding theminto a fractionating column, at a predetermined height level in thecolumn, to produce an upward-flowing vapor current and adownward-flowing liquid current therein.

[0018] Advantageously, the downward-flowing liquid current is dischargedcontinuously from the column bottom, and then fed continuously into theturbomixer in the same direction as the crude oil current.

[0019] The paddled rotor velocity is preferably of 15 to 40 m/s.

[0020] In an alternative embodiment of this invention, the turbomixermay be used to only blend together the steam and the downward-flowingliquid from the bottom of the fractionating column, and the crude oilmay be fed directly into the fractionating column.

[0021] In this case, the method of this invention would comprise thesteps of:

[0022] providing a turbomixer, which comprises a tubular cylindricalbody provided with a heating jacket, at least one inlet opening, and atleast one outlet opening, and comprises a rotor formed with helicallyoriented paddles and rotatably supported inside said body;

[0023] providing a fractional distillation column;

[0024] feeding a continuous current of crude oil, as preliminarilydesalted and heated to 310-400° C., into said fractionating column, at apredetermined height level in the column, to produce an upward-flowingvapor current and a downward-flowing liquid current therein.

[0025] continuously discharging said downward-flowing liquid currentfrom the bottom of said column and feeding it continuously into saidturbomixer in the same direction as a continuous steam current, with thewall of said turbomixer being heated to 310-400° C.;

[0026] subjecting both said currents to the mechanical action of saidpaddled rotor, as driven for a peripheral velocity of at least 15 m/s,so as to blend both currents together, centrifuging the blended currentsagainst said heated wall to create a thin dynamic tubular layer inturbulent motion thereon, and conveying said blended currents to said atleast one outlet opening; and

[0027] continuously discharging said blended currents, and continuouslyfeeding them into said fractionating column at said predetermined heightlevel thereof.

[0028] It has been found experimentally that, by having the crude oiland steam intimately blended together within the turbomixer, a moreeffective distillation can be obtained, and this at a lower operatingtemperature. The lower temperature additionally minimizes any undesiredcracking reactions.

[0029] By having the oil and steam instantly intimately contacted withinthe blend, a faster and more effective separation of the volatilecomponents is promoted, and the amount of recycling required for properfractionating is reduced.

[0030] Also, recycling the liquid phase collected from the column bottomallows those low-boiling components that, in conventional fractionatingprocesses, remain trapped in the residue on the column bottom to bereleased in the utmost proportion.

[0031] An important advantage of the inventive process is that it canavoid, or at least minimize, the formation of dense viscous residue onthe column bottom by virtue of the liquid on the column bottom beingcycled continuously back into the turbomixer.

[0032] The last-mentioned apparatus is made very nearly self-cleaning bythe powerful mechanical action of the rotor paddles, which graze theinner wall surface of the cylindrical body in the turbomixer, constantlywiping it clean.

[0033] In this way, the need to have the column bottom cleanedperiodically is eliminated, or at least greatly reduced, which, as saidbefore, considerably depressed the refining plant throughput.

[0034] Additional advantages of the process of this invention will beapparent from the following description of an embodiment thereof, givenhere below by way of non-limitative example with reference to theaccompanying drawing, which shows schematically an apparatus forcarrying out this process.

[0035] With reference to the drawing, an apparatus used for carrying outthe inventive process is of a type known as “turbomixer” (e.g. assupplied by VOMM Impianti e Processi, of Rozzano, MI, Italy). Itcomprises basically a tubular cylindrical body 1 that is closed ateither ends by end walls 2, 3 and provided with a coaxial heating jacketthrough which a fluid, e.g. a diathermic oil, is circulated.

[0036] The tubular body 1 has an inlet opening 5 for admitting crude oiland steam thereinto, and has an outlet opening 6 used for delivering theoil/steam blend, this outlet opening being communicated to afractionating column 8 by a conduit 7.

[0037] A paddled rotor 9 is rotatably supported inside the tubular body1. The paddles 10 of this rotor are helically arranged and oriented tocentrifuge the process currents and concurrently convey them to theoutlet. A motor 13 drives the rotor 9 at a peripheral velocity in therange of 15 to 40 m/s, preferably of 20 to 30 m/s.

[0038] In actual practice, the turbomixer could be formed with more thanone inlet openings, contingent on individual requirements.

[0039] The column 8 is more markedly schematized in the drawing becauseconventional. This column is in communication, as said before, with theturbomixer 1 over the conduit 7, which conduit extends from the outletopening 6 of the turbomixer to an opening 11 provided at a predeterminedheight level in the column 8. The column 8 is also communicated to theturbomixer 1 by a conduit 12 that extends from an opening 13 in thecolumn bottom to the inlet opening 5 of the turbomixer 1.

[0040] The process of this invention is conducted on this apparatus inthe manner described here below.

[0041] A current of crude oil, as preliminarily desalted and heated toabout 310-400° C. through a furnace, is fed continuously into theturbomixer 1 through the inlet opening 5. The turbomixer 1 is also fedcontinuously a steam current, concurrently with and in the samedirection as the oil current, again through the inlet opening 5. Theliquid and vapor phases that comprise the heated oil current are bothcentrifuged by the blades 10 of the rotor 9 against the heated innerwall of the turbomixer, directly as it enters the turbomixer along withthe concurrent steam infeed. The helical orientation of the blades 10 ofthe rotor 9 is also effective to convey the oil/steam blended current tothe discharge opening 6.

[0042] Throughout their flowpath in the turbomixer 1, the blendedcurrent is in the form of a thin tubular dynamic layer in turbulentmotion across the inner wall of the turbomixer toward the dischargeopening 6.

[0043] In this situation, a deep exchange of matter and energy occursbetween the crude oil and the steam, and a “pre-distillation” of sorttakes place, whereby the low-boiling components begin to separate and,most importantly, a blend of liquid and vapor phases is formed that canbe distilled on conventional fractionating columns under less harshconditions than are conventional.

[0044] This blend of the crude oil and steam currents flowing out of theturbomixer through the opening 6, after a residence time of 15 to 60seconds inside the turbomixer, is taken over the conduit 7 to thefractionating column 8, whereinto it is fed at a predetermined heightlevel in the column through the opening 11.

[0045] At this stage, known energy exchange phenomena occur at thevarious trays in the column, with repeated vaporizing and condensingsteps being carried out that lead to the components becomingfractionated. The most volatile components are condensed upon reachingthe column head, and in part re-sent into the column as a reflux, as iscustomary in crude oil fractionating operations. The interestingfractions (gasoline, Diesel oil, kerosene, etc.) can be withdrawn in thesame way at different height levels from the column.

[0046] On the other hand, the high-boiling liquid components flow downto and collect at the bottom of the column, whence they are dischargedthrough the opening 13 and fed back into the turbomixer over the conduit12, along with the crude oil.

[0047] In this way, the dense viscous residue that typically forms onthe bottom of conventionally operated fractionating columns and tends toinclude valuable low-boiling components no more releasable by merestripping with steam injected at the column bottom, thereby pulling downthe process throughput, is avoided.

[0048] Within the turbomixer, the density of the liquid being processedwith steam increases progressively and raises the loading on the drivemotor of the paddled rotor. Upon the motor amperage reaching apredetermined threshold value, the material contained in the turbomixeris discharged and optionally conveyed to a vacuum fractionating column,also equipped with a respective turbomixer, over a special conduit 14.

[0049] By virtue of the intimate contact established between the steamand the organic components inside the thin tubular turbulent layer onthe turbomixer interior—organic components from both oil newly fed intothe apparatus and the recycled current from the bottom of thefractionating column—the low-boiling components are effectively releasedat once, even after the recycled current from the column bottom hasbecome highly dense and viscous.

[0050] The process of this invention is further illustrated by makingreference to an example of a crude oil fractional distillation carriedout on a conventional fractionating column operated under atmosphericpressure. The process of this invention could, however, be carried outunder a vacuum, by using fractionating columns designed for low-pressureoperation and adjusting the experimental parameters in ways well knownto the persons skilled in the art.

EXAMPLE

[0051] Using the apparatus summarized hereinabove in the process of thisinvention, crude oil from a preheating furnace at 350° C. was fedcontinuously into turbomixer 1 at the rate of 1000 l/h, concurrentlywith and in the same direction as a steam current.

[0052] The wall temperature was maintained at about 350° C., and therotational velocity of the paddled rotor 9 constant at 30 m/s.

[0053] After an average 50 seconds of residence in the turbomixer, acurrent of steam, and vapor and liquid oil components, was continuouslydischarged and continuously fed into a fractionating column 8 underatmospheric pressure, at the level of an intermediate tray in thecolumn.

[0054] Once a condition of equilibrium was attained for distillation,the usual fractions (gasoline, Diesel oil, kerosene, etc.) werewithdrawn continuously at given levels, at the same time as the refluxrate was adjusted in conformity with ASTM Method D 2892. This Methodprovides for 14 to 17 theoretical trays in the column and a reflux ratioof 5. At steady state, a liquid current was discharged continuously fromthe column bottom and continuously returned to the turbomixer overconduit 12, at the same time as the rate of the fresh crude oil inflowto the turbomixer was reduced. Thus, the overall flow rate was kept at1000 l/h for both currents.

[0055] After 4 weeks of continuous operation, the process was halted toinspect the turbomixer conditions. The turbomixer was found completelyfree of any viscous tarry residue.

[0056] The process throughput showed to be improved as far as valuablelighweight components were concerned, and the high-boiling (above 310°C.) components were found of uncommon good quality.

1. A process for the fractional distillation of crude oil, whichcomprises the steps of: providing a turbomixer comprising a tubularcylindrical body (1) provided with a heating jacket (4), at least oneinlet opening (5), and at least one outlet opening (6), and comprising arotor (9) provided with helically oriented paddles (10) and rotatablysupported inside said body; feeding a continuous current of crude oil,as preliminarily desalted and heated to 310-400° C., into saidturbomixer, with the turbomixer wall being heated to the sametemperature as the inflow current of crude oil; supplying a steamcurrent continuously in the same direction as said crude oil current;subjecting both said currents to the mechanical action of said paddledrotor (9), as driven for a peripheral velocity of at least 15 m/s, so asto intimately blend both currents together, centrifuging the blendedcurrents against said heated wall to create a dynamic thin tubular layerin turbulent motion thereon, and conveying the blended currents to saidat least one outlet opening (6); and continuously discharging saidblended currents and feeding them into a fractionating column (8), at apredetermined height level in the column, to produce an upward-flowingvapor current and a downward-flowing liquid current therein.
 2. Aprocess according to claim 1, further comprising continuouslydischarging said downward-flowing liquid current from the bottom of saidcolumn (8) and continuously feeding it into said turbomixer in the samedirection as said crude oil current.
 3. A process for the fractionaldistillation of crude oil, which comprises the steps of: providing aturbomixer comprising a tubular cylindrical body (1) provided with aheating jacket (4), at least one inlet opening (5), and at least oneoutlet opening (6), and comprising a rotor (9) formed with helicallyoriented paddles (10) and rotatably supported inside said body;providing a fractional distillation column (8); feeding a continuouscurrent of crude oil, as preliminarily desalted and heated to 310-400°C., into said fractionating column (8), at a predetermined height levelin the column, to produce an upward-flowing vapor current and adownward-flowing liquid current therein. continuously discharging saiddownward-flowing liquid current from the bottom of said column (8) andfeeding it continuously into said turbomixer in the same direction as acontinuous steam current, with the wall of said turbomixer being heatedto 310-400° C.; subjecting both said currents to the mechanical actionof said paddled rotor (9), as driven for a peripheral velocity of atleast 15 m/s, so as to intimately blend both currents together,centrifuging said blended currents against said heated wall to create athin dynamic tubular layer in turbulent motion thereon, and conveyingthe blended currents to said at least one outlet opening (6); andcontinuously discharging said blended currents, and continuously feedingthem into said fractionating column (8) at said predetermined heightlevel.
 4. A process according to any of the preceding claims, whereinthe rotational velocity of said paddled rotor is of 15 to 40 m/s.
 5. Anapparatus for the fractional distillation of crude oil, comprising atleast one rectification column (8) and characterized in that said atleast one column (8) is in fluid communication with a turbomixercomprising a tubular cylindrical body (1) provided with a heating jacket(4), at least one inlet opening (5), and at least one outlet opening(6), and comprising a rotor (9) formed with helically oriented paddles(10) and rotatably supported inside said body.
 6. An apparatus accordingto claim 5, wherein said at least one inlet opening (5) of saidturbomixer is communicated over a conduit (7) to an opening (11)provided at a predetermined height level in said column (8).
 7. Anapparatus according to claim 6, wherein said at least one inlet opening(5) of said turbomixer is communicated over a conduit (12) to an opening(13) provided in the bottom of said column (8).